Oct 3 (Reuters) - Sarepta Therapeutics Inc said its experimental drug met the main goal of improving walking ability in patients with a rare muscle disorder in a mid-stage trial, sending its shares up 111 percent in premarket trade.

The drug eteplirsen significantly improved walking ability in a 48-week study involving patients with Duchenne Muscular Dystrophy (DMD) - a genetic disorder that causes severe and progressive muscle loss in boys. There are no approved treatments for DMD.

The mid-stage study compared two separate doses of the drug,

50mg/kg and 30mg/kg, with a placebo and the higher dose improved walking ability by 89.4 meters.

The improvement was measured by a standard test called the six-minute walk test that checks the cardiac, respiratory, circulatory, and muscular capacity of patients.

Sarepta's shares have more than tripled in value as of Tuesday after the drug showed improved walking ability according to trial data announced in late-July.

DMD is caused by a defective gene that codes for dystrophin, a protein that plays a key role in muscle fiber function.

Sarepta's drug works by trying to skip a particular section of the defective gene to repair the genetic mutations. The skipped section, exon 51, is responsible for the production of dystrophin.

(Reporting by Adithya Venkatesan in Bangalore; Editing by Sreejiraj Eluvangal)